Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy by oxidization of fuel supplied to the cells. Generally, a fuel cell comprises two gas diffusion electrodes adjacent to and in contact with an electrolyte, and includes a means for supplying fuel to the positive electrode and supplying an oxidant to the negative electrode. The electrolyte, either solid or liquid, is located between the electrodes and transfers ions between the positive and negative electrodes.
One type of fuel cell utilizes a proton exchange polymer film as the electrolyte. In this type of fuel cell, a plurality of acid functional groups chemically bonded to the polymer main chain of the polymer film have an electrolytic action. The proton exchange polymer film may be made of, for example, sulfonated polystyrene, or preferably a substantially fluorinated sulfonic acid polymer such as a Nafion ion exchange polymer film manufactured by Du pont. The term “solid polyelectrolyte” is often used to indicate any of these ion exchange polymer film structures.
Proton exchange polymer film fuel cells are known, and disclosed in, for example, U.S. Pat. No. 3,134,697. Early solid polyelectrolyte proton exchange polymer film fuel cells were operable, but had a limited life because of the chemical instability of the polymer film as such. However, subsequently developed perfluorinated polymer materials having ion exchange activity, such as Nafion manufactured by Du pont, enabled provision of fuel cells of the above type which have good operation properties and a several thousand hour life.
A solid polyelectrolyte fuel cell comprising Nafion (perfluorosulfonic acid polymer film) is operated generally at about 80° C. Since the polymer film as such is substantially gas-impermeable, the cell does not necessitate any porous support matrix for preventing mixing of gases, which is usually employed when using a liquid electrolyte fuel cell. By using a suitable external support, a pressure difference of 100 psi or more can be provided between the fuel gas and oxidant gas in practical operation. These characteristics are remarkably advantageous. When the fuel cell is operated using, as an oxidant, air with an increased pressure to raise the oxygen partial pressure, compression of the fuel gas becomes unnecessary. For example, a hydrogen/air fuel cell can be operated using fuel at 1 atmosphere and air (oxidant) at 4 atmospheres or more.
In practice, the electrodes are physically bonded to the active proton exchange polymer film by application of pressure or heat (e.g., U.S. Pat. No. 4,272,353).
In the present state of the art, perfluorosulfonic acid polymer films manufactured by Du pont, as described in U.S. Pat. No. 3,282,875, are used as films having an equivalent weight of about 1100 to 1200. Equivalent weight means the weight of a polymer necessary for neutralizing 1 equivalent of a base. It is presumed that the ion conductivity of a polymer film is in inverse proportion to the equivalent weight of the polymer film. There is a film of Nafion ion exchange polymer having a lower equivalent weight than the polymer film conventionally employed in the art (EP Patent Application No. 0122049). However, polymer films having an equivalent weight less than about 950 have low physical stability as described in “Dual Cohesive Energy Densities of Perfluoro-sulphonic Acid (Nafion) Membrane” (Polymer, vol. 21, pp. 432-435, April, 1980). Thus, such polymer films have handling difficulties during assembling of a cell unit, or produce creep during assembling or operation of a cell, thus causing voltage loss or short circuit and losing reliability. A proton exchange solid polyelectrolyte with a reduced equivalent weight is strongly desired in order to decrease the resistance loss of ion transfer in fuel cells while maintaining acceptable properties of the cells.
A variety of attempts have been made to solve the problems of solid polyelectrolyte films.
For example, International Patent Application Translation Publication No. 1987-500759 states that the structure of the Nafion ion exchange polymer having sulfonic acid groups in the polymer side chain is modified, i.e., the sulfonic acid group-containing side chain structure is made shorter. As a result, according to the publication, the polymer has a lower equivalent weight (less than 1000) and an improved storage elastic modulus at a high temperature (110° C. or higher). In this technique, the glass transition point or softening point of the Nafion ion exchange polymer (about 110° C.) is raised to improve the high-temperature mechanical properties. However, sulfonic acid group-containing polymers, including Nafion ion exchange polymers, are intrinsically amorphous or have an extremely low crystallinity even if they have a crystalline moiety. Therefore, these polymers are insufficient in mechanical properties at room temperature or high temperatures.
Further, it is very difficult to synthesize the disclosed sulfonic acid group-containing polymers whose sulfonic acid group-containing side chain is short. Therefore, such polymers are disadvantageous in productivity and cost.
Japanese Unexamined Patent Publication No. 1994-231778 discloses a blend of at least two perfluorocarbon polymers having sulfonic acid groups, which polymers are different in ion exchange capacity, for use in a solid polyelectrolyte fuel cell. According to the publication, the blend has high ion exchange capacity and contributes to high mechanical strength.
However, the publication discloses a mere blend of a perfluorocarbon polymer having a large amount of sulfonic acid group-containing units and a perfluorocarbon polymer having a small amount of sulfonic acid group-containing monomer units. Therefore, the mechanical strength of the blend is middle between the mechanical strengths of the unblended polymers. Moreover, the polymer with low ion exchange capacity (with small sulfonic acid group content), which contributes to the mechanical strength, do not have significantly high mechanical properties, and thus is not capable of giving the blend sufficient mechanical strength. Furthermore, it is difficult to homogeneously blend the polymers, thus resulting in impaired proton transfer properties.
Japanese Unexamined Patent Publication No. 1994-231781 describes a solid polyelectrolyte fuel cell comprising a laminate of at least two perfluorocarbon polymers containing sulfonic acid groups, which polymers are different in water content. In this technique, the perfluorocarbon polymer with a low water content (with a small amount of sulfonic acid group-containing monomer units) in the laminate is intended to contribute to high mechanical strength. However, the low water-content polymer as such has insufficient mechanical strength, and the layer of the polymer is thin, and therefore the laminate does not have sufficiently improved mechanical strength. Moreover, the laminated layers have a high water-content portion, and a low water-content having low ion transfer properties. Thus, the laminate as a whole has reduced proton transfer properties.
The present invention has been accomplished in view of the above problems of the prior art.
An object of the present invention is to provide a solid polyelectrolyte material having necessary and sufficient hydrogen ion conductivity (ion exchange group concentration) for use in a fuel cell comprising a sulfonic acid group-containing fluoropolymer, the material also having necessary and sufficient mechanical properties and durability for assembling, processing or use of the fuel cell.